Composite materials can be used to form very durable and lightweight structures. Composite materials generally have better strength-to-weight ratios than metals such as aluminum or steel. Composite materials can also be more readily formed into complex shapes and configurations. For at least these reasons, composite materials are competitive with, if not replacing, metal materials in the manufacture of items such as tennis racquets, golf clubs, bicycle frames, and parts for automobiles, aircraft, and spacecraft.
Composite materials are typically formed of fibers (e.g., glass and/or carbon fibers) embedded in a resin matrix. The fibers introduce anisotropic mechanical properties that can be used to optimize the weight and reduce the material usage for structures. In addition, advanced composites frequently use stiffening elements or hollow features that can be co-cured or co-bonded during the composite consolidation process, eliminating secondary fastening and bonding operations.
A common challenge for fabricating composite structures is the need for tooling that, while initially trapped in cavities within the composite structure, can be extracted after the curing process. Elastomeric tooling can be used that permits removal through openings in the composite structure due to the elastomeric tooling's ability to change cross-section when a force is applied, such as a vacuum or a pull force on an end. Elastomeric tooling can also allow pressure to be transferred in a controlled manner to a laminate or a surface of a composite structure, which can be critical for the composite fabrication process.
A technical problem related to elastomeric tooling is the gas permeability of the elastomeric material. The permeability increases with temperature and is significantly higher at the elevated temperatures used during composite manufacturing processes. At the same time, the less permeable elastomeric materials are not able to withstand repeated uses at high temperatures. The relatively soft elastomeric material can also be prone to mechanical damage. It is often the case that some or all of the elastomeric tooling presses against the composite structure during the curing process. In such processes, the elastomeric tooling may contain gas at a significantly higher pressure than the pressure between the composite structure and the outer surface of the elastomeric tooling. This is particularly true given that the composite structure may be under vacuum and the gas on the inside of the elastomer can be under pressure in a molding application. This can be problematic, in that it promotes gas permeation or flow through the elastomer to the composite due to the pressure differential between the gas on the inside of the elastomer and the outer surface of the elastomer on the composite side, thereby amplifying the risk of damage or quality issues of the composite material in the vicinity of the elastomeric tooling. Both the inherent gas permeability of the elastomeric material and/or possible damage to the elastomer allow gases to reach the composite and can potentially result in porosity, dry spots, and/or inadequate pressure on the composite during consolidation. Barrier layers of a low permeability material can be added to the elastomeric tooling to attempt to overcome some of these problems. However, barrier layers are generally mismatched in modulus as they are almost universally less elastic and have a different coefficient of thermal expansion than the elastomeric material they are attached to, which can lead to delamination during cure cycles and extraction. Even with such barrier layers, the elastomeric tooling is still prone to manufacturing defects and punctures, which limit the re-use of such tooling and raise the risk of gas permeation after repeated use.
Another technical problem related to elastomeric tooling is that it is difficult to discriminate between the natural permeability of the material and small punctures or other leaks. Leak check testing is often not reliable for finding small leaks as the leaks can close by self-sealing under pressure or under vacuum or be undetectable due to variation in the permeability of the elastomeric tooling. Also, the often large interior volume of the elastomeric tooling makes such checks very sensitive to changes in atmospheric pressure or ambient temperature.